Simple Interest Questions for SSC MTS

What is Simple Interest

When you borrow or lend money, the extra amount paid on top of the original money is called interest. In Simple Interest, the interest is always calculated on the original amount (Principal) only — it never changes year after year. Think of it like paying a fixed rent every year on the same house, regardless of anything else.

• Principal (P) = the original amount borrowed or invested
• Rate (R) = interest charged per year, expressed as a percentage
• Time (T) = number of years the money is borrowed or invested
• SI = the interest earned or paid, not including the Principal
• Amount (A) = Principal + Simple Interest

Amount vs Simple Interest — Do Not Confuse

Many SSC MTS questions give you the Amount (total money returned) and ask you to find SI or Principal. Remember: Amount is the full picture, SI is just the extra part. If A and P are given, SI = A minus P. If A and SI are given, P = A minus SI. This swap trick is tested very often.

• Amount = Principal + SI, so SI = Amount - Principal
• If question says 'a sum amounts to X in Y years at Z%', it means A = X
• Always extract P and SI separately before applying the formula
• Do not plug Amount into the SI formula in place of Principal — common trap

Rate or Time in Months and Days

Sometimes SSC MTS gives time in months instead of years. Always convert months to years by dividing by 12 before using the formula. Similarly if time is in days, divide by 365. The rate is always per year unless stated otherwise. Missing this conversion is a very common silly mistake.

• 6 months = 6/12 = 0.5 years
• 9 months = 9/12 = 0.75 years
• 3 months = 3/12 = 0.25 years
• Always check the unit of time in the question before solving
• Rate is per annum (per year) by default in all SSC questions

Shortcut — SI as Fraction of Principal

For quick calculation, express SI as a fraction of P. If R = 10% and T = 3 years, SI = 30% of P = 0.30P. This avoids division and speeds up calculation in MCQ format. Practice expressing common Rate x Time products as simple percentages.

• R x T gives the total percentage of SI on Principal
• 10% for 3 years = 30% of P as SI
• 5% for 4 years = 20% of P as SI
• 8% for 5 years = 40% of P as SI
• This trick is fastest when options are in round numbers

• Using Amount (A) instead of Principal (P) in the SI formula — always separate them first
• Forgetting to convert months to years — 6 months is 0.5 years, not 6
• Confusing SI with Amount when the question asks 'how much interest is earned' versus 'what is the total amount'
• Calculating R x T wrongly when rate or time has decimals — write it out step by step
• When two parts of a Principal are invested at different rates, students add rates instead of calculating SI separately for each part and then adding

• PRT formula mnemonic: 'Please Remember This divided by 100' — P x R x T divided by 100 = SI
• A = P + SI is like your wallet: Amount is what you get back, Principal is what you put in, SI is the bonus
• Months trick: think of 12 months in a clock face — divide time by 12 to get years
• To find any missing value (P, R, or T), cover it in the triangle: SI on top, P-R-T at base — whatever is covered, it equals the other two multiplied, then divide by 100

• SSC MTS typically has 1 to 2 questions per paper on SI, usually finding SI directly or finding P when Amount is given — both are one-step calculations
• Questions rarely go beyond 2-step logic in MTS — if your calculation is becoming complex, recheck if you used Amount instead of Principal
• Time is often given in months in recent MTS papers to create a conversion trap — always read units carefully
• Options in MTS SI questions are usually round numbers like 600, 750, 1200 — if your answer is odd or fractional, recheck your T in years
• Attempt SI questions first in the Quant section — they take under 60 seconds each and build confidence for harder topics